Wireless communication devices have become smaller and more powerful as well as more capable. Increasingly users rely on wireless communication devices for mobile phone use as well as email and Internet access. At the same time, devices have become smaller in size. In addition, devices may now incorporate multiple transmitters and antennas. These factors may increase a user's exposure to radio frequency (RF) radiation. Devices such as cellular telephones, personal digital assistants (PDAs), laptop computers, and other similar devices provide reliable service with expanded coverage areas. Such devices may be referred to as mobile stations, stations, access terminals, user terminals, subscriber units, user equipments, and similar terms.
A wireless communication system may support communication for multiple wireless communication devices at the same time. In use, a wireless communication device may communicate with one or more base stations by transmissions on the uplink and downlink. Base stations may be referred to as access points, Node Bs, or other similar terms. The uplink or reverse link refers to the communication link from the wireless communication device to the base station, while the downlink or forward link refers to the communication from the base station to the wireless communication devices.
Wireless communication systems may be multiple access systems capable of supporting communication with multiple users by sharing the available system resources, such as bandwidth and transmit power. Examples of such multiple access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, wideband code division multiple access (WCDMA) systems, global system for mobile (GSM) communication systems, enhanced data rates for GSM evolution (EDGE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Wireless devices, including mobile telephones are required to undergo testing to determine the amount of RF energy a user may be exposed to when using the device. In the U.S., the Federal Communications Commission (FCC) certifies mobile devices to ensure compatibility with requirements and user safety. This FCC certification requires time consuming spatial measurements, which are made by attaching devices to a phantom filled with tissue simulating liquid that simulates the response of a human body. The FCC requires evaluation of the peak (1 g volume) averaged SAR. This testing involves measuring the electric field in a three dimensional human head/body phantom volume filled with tissue simulating liquid.
Since it is time consuming to perform the three-dimensional scan over the entire region exposed to the radiation from the wireless device, the measuring time is reduced by a first measuring the electric field along the two-dimensional surface contour of the head/body phantom. This is known as an “area scan.” The area scan is followed by a “volume scan” around the peak electric field (SAR) location, which was previously identified by the area scan. Typically the volume scan takes more time to measure, but provides more detail about the local SAR location. In the volume scan, averaging is performed over 1 g-volume of tissue to determine the peak 1 g-averaged SAR, which is used to demonstrate compliance with the SAR limits, as required by the regulatory body whose certification is sought.
The procedure described above is suitable for measuring SAR from wireless devices with only one transmitter. When multiple transmitters are present on a device and are simultaneously active, it is not practical to combine the SAR measurements from individual transmitters. This is because typically the peak SAR location determined from the area scan is at different locations for each of the multiple transmitters. As a result, there is no overlap in the smaller localized volume scans performed when evaluating peak 1 g-averaged SAR for the individual transmitters. Presently, the only alternative to overcoming this deficiency is to perform an over-sized volume scan that covers the entire region exposed to the wireless device. This approach is extremely time consuming Due to the lack of alternatives, device manufacturers are adding the peak 1 g-averaged SAR values from the individual transmitters to produce a combined SAR value for the device. This approach results in an overly conservative value for the device, which in turn, leads to significant back off in transmit power in order to gain certification. There is a need in the art for a method and apparatus to combine the SAR readings from multiple simultaneous transmitters.